A pressure cooker is a receptacle, e.g. a deep pan, which is hermetically sealed by a lid provided with a special clamping device, and within which steam that is obtained by boiling water raises the internal temperature to higher than 100.degree. C. so as to shorten ordinary cooking time.
The pressure regulator associated with the lid is designed to maintain the pressure inside the pressure cooker at a reference value and to exhaust excess steam that would otherwise tend to increase the pressure to a value above the reference. This serves to regulate the cooking temperature.
In a conventional design, the regulator is of the type comprising a valve body designed to be fixed to the lid of the pressure cooker and in which a valve seat is formed that co-operates by gravity with a valve closure element in particular in the form of a needle, which carries a rating mass that is mounted to slide vertically relative to the valve body.
An appropriate choice of rating mass determines the reference value, i.e. the pressure that will be established inside the pressure cooker and above which the assembly constituted by the rating mass and the closure element rises relative to the valve body, thereby opening the valve and discharging excess pressure.
In certain applications, it has been found necessary for the user to be able to adjust the value of the pressure that exists inside the pressure cooker, and in particular to be able to select, e.g. between at least two pressure values, in order to improve performance and the way in which the pressure cooker is used by selecting cooking temperature and/or cooking duration.
In a first known design, the user may select the reference value by selecting from two or more rating masses, with the selection directly determining cooking temperature.
That solution is satisfactory from the functional point of view, but it suffers from the drawback of providing at least two rating masses together with the pressure cooker, each mass including its own closure needle.
In addition, if the user observes that the wrong rating mass has been selected initially, then it is necessary for the pressure inside the pressure cooker to be exhausted completely or in part by removing the wrong needle prior to being able to install a different needle.
In a second design described and shown in document GB-A-2 115 524, a regulator is proposed that is constituted by a closure element that slides in a plug that is screwed to the body of the safety valve and that is urged resiliently in the closure direction by a rating spring which bears against the plug and against a shoulder of the closure element.
The bottom end of the closure element is a cylindrical rod that slides in a cylindrical bone in the valve body, and the initial setting for the rating value as exerted by the spring defines the strong beyond which the safety valve opens, thereby determining the rating of the regulator.
That design of regulator suffers from the drawback of requiring a lengthy and fiddly unscrewing operation when the user desires to exhaust all of the pressure inside the pressure cooker.
In addition, the top end of the closure element projects axially above the screw plug and it is possible to alter the setting of the rated pressure by accidentally pressing on the free end of the rod, and this may go so far as to prevent the regulator from operating at all. Finally, there is a risk of the bottom end of the rod becoming jammed in its bore.
Document EP-A-0 420 324 describes and shows a design in which the rating mass is provided with a pair of valve seats and associated closure elements.
To this end, the rating mass is of a particularly complex structure having two portions mounted to move relative to each other, each of which includes one of the needle-shaped closure elements.
As well as being complex in structure, that design also suffers from the drawback of requiring the two portions of the rating mass to be accurately positioned relative to each other in order to avoid the rated value of the regulator being indeterminate, i.e. situated in a range defined by the lower and upper rating values that are associated with operation of one or the other of the seat and closure element assemblies.